Strain Stiffening and Negative Normal Stress in Alginate Hydrogels
Hashemnejad, S. M., & Kundu, S. (2016). Strain Stiffening and Negative Normal Stress in Alginate Hydrogels. Journal of Polymer Science, Part B: Polymer Physics. Wiley. 54, 1767-1775. DOI:10.1002/polb.24081.
Alginate hydrogels are polysaccharide biopolymer
networks widely useful in biomedical and food applications.
Here, we report nonlinear mechanical responses of ionically
crosslinked alginate hydrogels captured using large amplitude
oscillatory shear experiments. Gelation was performed in situ
in a rheometer and the rheological investigations on these
samples captured the strain-stiffening behavior for these gels
as a function of oscillatory strain. In addition, negative normal
stress was observed, which has not been reported earlier for
any polysaccharide networks. The magnitude of negative normal
stress increases with the applied strain amplitude and can
exceed that of the shear stress at large-strain. Fitting a constitutive
relationship to the stress-strain curves reveals that the
mode of deformation involves stretching of the alginate chains
and bending of both the chains and the junction zones. The
contribution of bending increases near saturation of G blocks
as Ca21 concentration was increased. The results presented
here provide an improved understanding of the deformation
behavior of alginate hydrogels and such understanding can be
extended to other crosslinked polysaccharide networks.